Warp triplet composite forming fabric

ABSTRACT

A forming fabric having a paper side layer and a machine side layer comprises a first set of paper side layer wefts, a second set of machine side layer wefts and a single set of warp yarn triplets. In the fabric weave pattern, each member of each triplet set of warp yarns interweaves with the paper side weft yarns to occupy in sequence segments of an unbroken warp path in the paper side surface, and each triplet in each set of warp yarns interlaces alone with at least one single machine side layer weft yarn. Each segment in the unbroken warp path is separated by at least one paper side layer weft yarn. The machine side layer interlacing points are regularly spaced. After heat setting, the fabrics typically have an air permeability typically from about 7,500 to about 10,500 m 3 /m 2 /hr. Paper products made using these fabrics have enhanced printability.

FIELD OF THE INVENTION

The present invention relates to woven forming fabrics for use inpapermaking machines. The forming fabrics of this invention consistessentially of at least two layers or sets of weft yarns, one in thepaper side layer of the fabric and the other in the machine side layerof the fabric, which are held together by one set of warps, which arewarp yarns woven in sets of three or triplets. Thus although visuallythe fabrics of this invention contain at least two layers, these are notseparate, interconnected woven structures, and cannot be separated intotwo distinct self-sustaining woven structures.

BACKGROUND OF THE INVENTION

The known composite forming fabrics comprise two essentially separatewoven structures, each of which includes its own sets of warps andwefts, and each of which is woven to a pattern selected to optimise theproperties of each of the layers. The paper side layer should provide,amongst other things, a minimum of fabric wire mark to, and adequatedrainage of liquid from, the incipient paper web. The machine side layershould be tough and durable, provide a measure of dimensional stabilityto the forming fabric so as to minimize fabric stretching and narrowing,and be sufficiently stiff to minimize curling at the fabric edges.Numerous fabrics of this type have been described, and are in industrialuse.

The two layers of the known composite forming fabrics are interconnectedby means of either additional binder yarns, or intrinsic binder yarns.Additional binder yarns serve mainly to bind the two layers together;intrinsic binder yarns both contribute to the structure of the paperside layer and also serve to bind together the paper and machine sidelayers of the composite forming fabric. The paths of the binder yarnsare arranged so that the selected yarns pass through both layers of thefabric, thereby interconnecting them into a single composite fabric.

In these known composite fabrics, additional weft binder yarns weregenerally preferred over intrinsic weft binder yarns, as they werebelieved to cause fewer discontinuities in the paper side surface of thecomposite fabric. Recently, both single and paired intrinsic warp orweft binder yarn arrangements have been proposed. However, intrinsicweft binder yarns have been found to cause variations in thecross-machine direction mesh uniformity. Composite fabrics in whichintrinsic weft binder yarns are incorporated have been found to besusceptible to lateral contraction under the tensile load placed uponthem in a papermaking machine. These intrinsic weft binder yarns havealso been found to be susceptible to internal and external abrasion,leading to catastrophic delamination of the composite fabric. Further,due to the necessity of having to weave into the fabric structureadditional weft yarns to form the paper side layer, and to bind thepaper side layer and machine side layer together, these fabrics areexpensive to produce.

More recently it has been proposed to use intrinsic warp binder yarns inpairs or triplets, so as to overcome at least some of thesedisadvantages. Fabrics of these two types are described by Vöhringer inU.S. Pat. No. 5,152,326 (pairs); by Stone et al. in U.S. Pat. No.6,240,973 (triplets); and by Johnson et al. in U.S. Pat. No. 6,202,705(triplets).

The use of pairs offers the advantages that the two warp binder yarnscan be incorporated in sequence in successive segments of an unbrokenwarp path in the paper side surface, and that there is more flexibilityof choice for the locations at which each member of the pair interlaceswith the machine side layer wefts. It is thus possible to optimise thepaper side surface to some extent, for example to reduce wire marking ofthe incipient paper web, and to improve the machine side layer wearresistance of the fabric, essentially by increasing the amount ofmaterial available to be abraded away before catastrophic failure,usually by delamination, occurs. In these fabrics using pairs of warpbinder yarns, the paper side layer and machine side layer each haveseparate weft yarn systems, one of which completes the paper side layerweave, and the other of which completes the machine side layer weave.

In the following discussion of this invention, it is to be understoodthat in a notation such as “2×2” the first number indicates the numberof sheds required to weave the pattern, and the second number indicatesthe number of wefts in the pattern repeat. Thus a 2×2 pattern requirestwo sheds, and there are two wefts in the pattern repeat.

As disclosed by Stone et al. and by Johnson et al. the use of warptriplets offers the advantage that the fabric structure can besimplified, in that the fabric can be woven with only three sets ofyarns: a paper side layer set of wefts, a machine side layer set ofwefts and a single set of warps which contributes to the structure ofboth layers. It is possible to weave a fabric having acceptable papermaking properties by utilizing triplets of warp yarns so that eachmember of the triplets interweaves separately in sequence with the paperside layer wefts, and so that the members of the triplets interlace inpairs with the machine side layer wefts. The pairs of warp yarns wheninterlaced with the machine side layer weft yarns cause these yarns tobow outwards somewhat, towards the machine side surface of the fabric.This provides a wear plane which increases fabric wear potential, whichincreases fabric life.

The use of triplets woven in pairs with the machine side layer weftsprovides a forming fabric having reduced susceptibility to cross-machinedirection variations in the paper side layer mesh uniformity, lesssusceptibility to dimpling of the paper side surface, and betterresistance to lateral contraction than comparable fabrics of the priorart. It is possible to weave some of these warp triplet fabrics from asingle warp beam, because all of the warp yarns follow essentiallysimilar paths, which have equal path lengths within the weave structure.

However it has been found that composite forming fabrics woven usingtriplet sets of warp yarns are still susceptible to dimpling of thepaper making surface of the paper side layer. It appears that, as thewarp yarns pass from one surface of the fabric to the other, eg from thesurface of paper side layer to the surface of the machine side layer,they may introduce some non-uniformity into the otherwise regularspacing of the paper side layer weft yarns. This creates variations inboth the shape and frame lengths of the drainage openings in the paperside layer of the forming fabric, which results in variation of thedrainage properties of the fabric. These variations can introduce anunacceptable level of marking (so-called “wire mark”) into the paperproduct being made.

It has now been found that this level of variation can be at leastmitigated by interlacing each member of a warp triplet set on its ownwith a machine side layer weft yarn. When this step is taken, it is thenpossible for each member of each of the triplet sets to follow the samepath within the weave pattern, thus providing more uniform location ofthe interlacing points. This has the result that the paper side layersurface characteristics are improved which in its turn provides moreuniform formation in the paper product.

Additionally, we have found that by careful choice of the warp yarnmaterial used in the fabrics of this invention, it is possible to weavea fabric having a high paper side surface open area with sufficientdrainage area to rapidly drain the embryonic sheet into the centralplane of the fabric structure, without sacrificing critical mechanicalproperties of the fabric, in particular its elastic modulus. In thecentral plane and machine side layer of the fabric, where yarn densityis higher, fluid drainage appears to be retarded slightly, thusproviding opportunity for pressure pulses caused by the supporting foilsand blades of the forming section to maximise formation benefits.

We have found that relatively smaller diameter, high elastic modulusyarns can be used in place of conventional, relatively larger diameterpolyethylene terephthalate(PET) yarns as the warp yarns in the fabricsof this invention, to provide equivalent mechanical strength properties.It is thus possible to use these smaller diameter yarns to provide thefabric with a relatively high paper side layer drainage area at a lowerwarp yarn density in the paper side surface. This in turn allows for theuse of a higher number of cross machine direction weft yarns than wouldotherwise be possible in the paper side surface so as to increase fibersupport in the sheet, thereby improving formation. These extra weftyarns will in turn contribute to overall fabric stiffness and stabilitywhich are necessary for dependable service life (i.e. “runnability”)

The fabrics of this invention are thus able both to drain fluid from thesheet more rapidly than would be possible in comparable fabrics wovenusing larger warp yarns, and to provide increased support for thepapermaking fibers in the stock so as to improve overall formation. Useof these high elastic modulus yarns also improves the resistance of thefabrics to damage from high pressure showers such as are used to cleanthem during use. Further, these smaller diameter, high elastic moduluswarp yarns will be recessed to an extent into the machine side surfaceof the fabric due to both machine side layer weave design and theheatsetting conditions used to process the fabric following weaving.Following heatsetting, the weft yarns on the machine side of the fabrictend to bow, or crimp outwardly, forming a wear plane which serves toprotect the warp yarns from abrasion during use. This feature serves toincrease further the service life of these fabrics.

SUMMARY OF THE INVENTION

In a first broad embodiment the present invention seeks to provide acomposite forming fabric having a paper side layer and a machine sidelayer, which comprises:

-   -   (i) a first set of paper side layer weft yarns,    -   (ii) a second set of machine side layer weft yarns which are        larger than the paper side layer weft yarns, and    -   (iii) a set of triplet warp yarns which contribute to the        structure of both the paper side layer and the machine side        layer,        which three sets of yarns are woven together according to a        repeating pattern wherein:

(a) each member of each triplet set of warp yarns interweaves with thepaper side layer weft yarns to occupy in sequence segments of a singleunbroken warp path in the paper side layer;

(b) the sequence of segments repeats as part of the repeating pattern;

(c) each segment in the unbroken warp path is separated next segment byat least one paper side layer weft yarn;

(d) each member of each triplet interlaces separately with a singlemachine side layer weft yarn at least once within the pattern repeat;

(e) within the fabric repeating pattern the number of machine side layerweft yarns between each interlacing point of successive yarns from eachtriplet of warp yarns is constant; and

(f) within the fabric repeating pattern the path lengths of each memberof each triplet set is the same.

In a preferred embodiment of this invention, the fabric as woven andprior to heat setting has a warp fill of from 100% to 125%.

In the forming fabrics of this invention, thermoplastic monofilamentsare used for both the warp yarns and the weft yarns.

In a first embodiment, the first and second set of weft yarns and thewarp yarns are all monofilaments of the same thermoplastic. Preferably,the warp yarns and the first and second sets of weft yarns are allpolyethylene terephthalate monofilaments.

In a second embodiment, the first set of weft yarns, the second set ofweft yarns and the warp yarns are not all monofilaments of the samethermoplastic.

In a third embodiment, the first set of weft yarns comprises at least afirst and a second subset of weft yarns and each subset comprisesmonofilaments of different thermoplastics.

In a fourth embodiment, the second set of weft yarns comprises at leasta third and a fourth subset of weft yarns and each subset comprisesmonofilaments of different thermoplastics.

In a fifth embodiment, the warp yarns are thermoplastic monofilamentshaving a higher modulus of elasticity than the paper side layer weftyarn thermoplastic monofilaments. Preferably the ratio of the moduli ofelasticity of the warp yarns and the paper side layer weft yarns isabout 4:3.

Preferably, within each of the first set of weft yarns, the second setof weft yarns, and the warp yarns, the yarns are all of the same size.

Preferably, the first set and the second set of weft yarns arepolyethylene terephthalate monofilaments.

Preferably, the second set of weft yarns are yarns chosen from the groupconsisting of polyethylene terephthalate monofilaments, monofilaments ofa blend of polyethylene terephthalate and a thermoplastic polyurethane;polyamide monofilaments and mixtures thereof. More preferably, in thesecond set of weft yarns the third subset comprises monofilaments of ablend of polyethylene terephthalate and a thermoplastic polyurethane,the fourth subset are yarns chosen from the group consisting ofpolyethylene terephthalate monofilaments, polyamide monofilaments andmixtures thereof, and the third subset comprises at least 50% of theyarns in the second set in the machine side layer.

Preferably, the warp yarns are chosen from the group consisting ofpolyethylene terephthalate monofilaments, polyethylene naphthalatemonofilaments, and mixtures thereof.

Preferably, the warp yarns are chosen from the group consisting ofpolyethylene naphthalate monofilaments, polyethylene terephthalatemonofilaments and mixtures of polyethylene naphthalate monofilaments andpolyethylene terephthalate monofilaments.

Preferably, the polyamide monofilaments are polyamide-6 or polyamide-6/6monofilaments.

In further preferred embodiments of this invention, the fabric afterheat setting has a paper side layer having an open area, when measuredby a standard test procedure, of at least 35%, the fabric has a warpfill of from 100% to 110%, and the fabric has an air permeability, whenmeasured by a standard test procedure, of from less than about 10,500m³/m²/hr, to as low as about 3,500 m³/m²/hr at a pressure differentialof 127 Pa through the fabric. An appropriate test procedure fordetermining fabric air permeability is ASTM D 737-96. Paper side layeropen area is determined by the method described in CPPA Data Sheet G-18using a plan view of this layer of the fabric.

It is a requirement of this invention that every warp yarn comprises atriplet of warp yarns; each member of each triplet in turn occupies aportion of an unbroken warp path in the paper side surface weave patternwhich repeats within the fabric weave pattern. Within the forming fabricoverall weave pattern, each member of each of the triplet warp yarnspasses alone into the machine side layer to interlace with at least onemachine side layer weft, so as to form a single coherent fabric. Theinterlacing locations are knuckles formed by the interlacing of theseparate members of each of the triplets with machine side layer weftyarns, so that within the fabric weave pattern repeat all three membersof each triplet interlace at least once with a machine side layer weft.The number of interlacing points within the weave pattern repeat isdetermined by the shed combination required for the individual weavepatterns chosen for the paper side layer and the machine side layer. Thelocation of interlacing points is chosen so that they are regularlyspaced within the machine side layer, with the same number of machineside layer weft yarns between each interlacing point.

In the preferred embodiments of this invention the warp monofilamentyarns and machine side layer weft monofilament yarns are fabricated fromdifferent thermoplastics. For example, polyethylene terephthalate, whichis commonly used in weaving forming fabrics, provides monofilaments withan elastic modulus of from about 1,400 kg/m² to about 1,550 kg/m²,whereas polyethylene naphthalate provides monofilaments with an elasticmodulus of about 2,000 kg/m². This ratio in the moduli of about 4:3 hasbeen found particularly advantageous.

The combinations of thermoplastic yarn materials in Table 1 have beenfound to be suitable.

TABLE 1 Combination Warp First Weft Second Weft A PET PET PET B PEN PETPET C PET PET PET/TPU D PET PET PET/TPU + PA6 E PET PET PET/TPU + PET FPEN PET PET/TPU G PEN PET PET/TPU + PA6 H PEN PET PET/TPU + PET I PENPET PET + PA6 J PET PET PET + PA6 Notes to Table 1. PET: polyethyleneterephthalate. PEN: polyethylene naphthalate. PEN/TPU: polyethyleneterephthalate modified with thermoplastic polyurethane (see Bhatt etal.) PA6: polyamide-6.

In Table 2 when mixtures of two yarns are identified, eg for CombinationD, it is preferred that the two yarns alternate.

When this combination of yarns with differing moduli of elasticity isused, it has been found that the relatively higher modulus warp yarnscan be woven into the fabric structure so as to impart sufficient crimpto the machine side layer weft yarns to cause them to bow outwardly fromthe plane of the machine side layer. By careful selection of the heatsetting conditions after weaving, the crimp imparted to the machine sidelayer weft yarns can be enhanced, which serves to recess the warp yarnsinto the structure of the fabric and thus protect them from abrasivewear. This step also allows each member of each triplet set to followmore or less the same path within the fabric weave structure, whichassists in reducing variations in the paper side layer mesh, therebyreducing any tendency for the fabric to cause wire mark.

It is thus apparent that in the fabrics of this invention the weft yarnscan be made to bow towards the various structures which support theforming fabric in a papermaking machine forming section. This creates awear plane on the machine side of the forming fabric. When the machineside layer weft yarns include a relatively highly abrasion resistantmonofilament, such as the polyethylene terephthalate—thermoplasticpolyurethane materials described by Bhatt et al. in U.S. Pat. No.5,169,171 and in U.S. Pat. No. 5,502,120, or a polyamide such aspolyamide-6 and polyamide-6/6, the fabric will be more resistant towear, and have a longer service life, than a comparable fabric wovenwithout these machine side layer weft yarns.

Although the fabrics of this invention can utilise differentthermoplastic monofilaments in each of the first set of wefts, thesecond set of wefts, and the warp, within each group of yarns all of theyarns are preferably the same size. It is also preferred that in orderto obtain as uniform a paper making surface as possible, the warp yarnsand the first set of weft yarns used in the paper side layer should alsobe substantially the same size.

In the fabrics of this invention neither the paper side layer nor themachine side layer contains any conventional warp yarns which interlaceonly with paper side layer weft yarns, or with machine side layer weftyarns. In the fabrics of this invention, a first group of wefts in thepaper side layer, and a second group of wefts in the machine side layer,are held together within the overall weave repeating pattern by a singleset of triplet warp yarns, which therefore contribute to both thestructural integrity and the properties of both layers.

The length of the segments in the paper side surface unbroken warp pathoccupied in sequence by each member of the triplets of warp yarns, andthe number of segments within one weave pattern repeat, are each open toa wide range of choices. For example, in fabrics discussed below in moredetail, both use weave patterns with six segments, in which the pathoccupied in the weave pattern repeat by each member of the triplets isessentially the same. In the unbroken warp path in the paper side layereach segment will generally occur in sequence more than once, forexample at least twice, within each complete repeat of the formingfabric weave pattern.

Preferably, each segment in the unbroken warp path in the paper sidesurface of the paper side layer is separated from an adjacent segment byeither 1, 2 or 3 paper side layer weft yarns. Preferably, each segmentin the unbroken warp path in the paper side surface of the paper sidelayer is separated from an adjacent segment by one paper side layer weftyarn. Alternatively, each segment in the unbroken warp path in the paperside surface of the paper side layer is separated from an adjacentsegment by two paper side layer weft yarns.

Preferably, within the paper side layer weave pattern, the total segmentlength or lengths occupied by each member of a triplet of warp yarnsoccupying the unbroken warp path are identical.

Since the paths occupied by each member of a triplet of paper side layerwarp yarns within the fabric weave pattern are essentially the same, andthe interlacing points between the warp yarns with the machine sidelayer wefts are regularly spaced, the composite forming fabrics of thisinvention will generally be woven using a single warp beam.

Preferably, the paper side layer weave pattern is chosen from a 2×2,3×3, 3×6 or 4×8 weave design. More preferably the paper side layer weaveis chosen from a plain 2×2 weave; a 3×3 weave; and a 4×4 weave.Preferably, the weave design of the machine side layer is chosen from a3×3, 4×4, 4×8, 5×5, 6×6 or 6×12 weave design. More preferably the weavedesign of the machine side layer is chosen from a 3×3 twill, a 6-shedbroken twill, a 9×9 twill or an N×2N design such as is disclosed byBarrett in U.S. Pat. No. 5,544,678. Most preferably, the weave design ofthe machine side layer is a 9×9 twill.

Preferably, the ratio of the number of paper side layer weft yarns tomachine side layer weft yarns is chosen from 1:1, 2:1, 3:2, 5:3, or 3:1.More preferably, the ratio is 2:1.

Due to the unique structure of the fabrics of this invention, it is notpossible to define a ratio of paper side layer warp yarns to machineside layer warp yarns. Only one member of a triplet appears at a time inthe paper side layer, and only one member of a triplet set appears at atime in the machine side layer. The fabric thus appears to have a 1:1warp ratio, but this is not meaningful in the context of these fabrics.

In the fabrics of this invention, selection of the paper side layerdesign and the machine side layer design must meet two criteria: first,in each repeat of the paper side layer weave design, each member of eachtriplet set of warp yarns interweaves in the paper side layer to occupyin sequence the segments of the unbroken warp path, and second in themachine side layer each member of each triplet interlaces alone at leastonce with a weft yarn in each repeat. This can be achieved by ensuringthat quotients which can be expressed as Q/P and Q/M, in which Q is thetotal number of sheds, P is the number of sheds required to weave thepaper side layer design, and M is the number of sheds required to weavethe machine side layer design. Q, M and P are always integers. Forexample, if P=2 and M=9 then Q=18 so that Q/P=9 and Q/M=2.

In the simplest embodiments, the fabrics of this invention will be wovenaccording to weave patterns requiring a loom equipped with at least sixsheds. This will accommodate a plain weave pattern for both the paperside layer and the machine side layer, and will require threerepetitions of the pattern to accommodate each of the three members ofthe triplets. However, such a simple embodiment is not generallypreferred, as machine side layer wear resistance of the resulting fabricmay not be adequate for most applications.

In the preferred embodiments of this invention, either a 2×2 plainweave, or a 3×3 twill weave is used for the paper side layer, combinedwith a 6-shed twill, a 6-shed broken twill, a 9×9 twill or an N×2N weavedesign for the machine side layer. The combination of a 2×2 plain weavewith a 6×6 twill will require 18 sheds: the 6×6 twill will require 18,and the 2×2 plain weave will require 6, thus giving quotients of 1 and 3respectively.

Table 2 summarizes some of the possible paper side layer and machineside layer weave pattern combinations, together with the shedrequirements for each.

TABLE 2 PSL PSL MSL MSL Total Quotient Weave Sheds, P Weave Sheds, MSheds, Q Q/P, Q/M 2x2 6 6x6 18 18 3, 1 2x2 6  6x12 18 18 3, 1 2x2 2 9x99 18 9, 2 3x3 9  6x12 18 18 2, 1 3x6 9  6x12 18 18 2, 1 2x2 6 4x4 12 122, 1 2x2 6 4x8 12 12 2, 1 3x3 9 4x4 12 36 4, 3 4x8 12 4x4 12 12 1, 1 4x812 4x8 12 12 1, 1 4x8 12 4x8 12 12 1, 1 2x2 6 5x5 15 30 5, 2 3x3 9 5x515 45 5, 3

In the headings to Table 2, “PSL” indicates paper side layer number ofsheds P, “MSL” indicates machine side layer number of sheds M, “TotalSheds” indicates the minimum number of sheds Q required to weave thefabric, and Q/P, Q/M are the integer values of the quotients of thenumber of the sheds required for the paper side layer divided into thetotal sheds, and the number of sheds required for the machine side layerdivided into the total sheds respectively.

Because all of the triplets of warp yarns making up the paper side layerwarp yarns are utilized to interlace with machine side layer weft yarns,this interlacing pattern improves fabric modulus, thus making the fabricmore resistant to stretching and distortion, while reducing lateralcontraction and any propensity for fabric layer delamination.

An important distinction between prior art fabrics and those of thepresent invention is the total warp fill, which is given by warpfill=(warp diameter×mesh×100) %. Warp fill can be determined eitherbefore or after heat setting, and, for the same fabric, is generallysomewhat higher after heat setting. In all prior art composite fabrics,prior to heat setting, the sum of the warp fill in the paper side andmachine side layers combined is typically less than 95%. The fabrics ofthis invention prior to heat setting can have a total warp fill thatpreferably is about 100%. After heat setting, the fabrics of thisinvention have a total warp fill that can be greater than 105%, and istypically about 110% or more.

In the context of this invention certain definitions are important.

The term “unbroken warp path” refers to the path in the paper sidelayer, which is visible on the paper side surface of the fabric, of thetriplets of warp yarns, and which is occupied in turn by each member ofthe triplets making up the warp yarns. This path continues along thefabric as the fabric weave pattern repeats.

The term “segment” refers to the portion of the unbroken warp path inthe paper side layer repeating pattern occupied by a specific warp yarn,and the associated term “segment length” refers to the length of aparticular segment, and is expressed as the number of paper side layerweft yarns with which a member of a triplet of warp yarns interweaveswithin the segment.

The term “float” refers to a yarn which passes over a group of otheryarns without interweaving with them; the associated term “float length”refers to the length of a float, expressed as a number indicating thenumber of yarns passed over.

The term “internal float” has a similar meaning and refers to thatportion of a yarn which passes between the layers of a composite fabricfor a short distance following interweaving with the paper side layer orinterlacing with the machine side layer. The associated term “internalfloat length” refers to the number of yarns from either the paper sidelayer or the machine side layer, as appropriate, between the two ends ofan internal float.

The term “interlace” refers to a point at which a single member of atriplet of warp yarns wraps alone about a machine side weft to form asingle knuckle, and the associated term “interweave” refers to a locusat which a single member of a triplet wraps about one or more paper sidelayer weft yarns and forms either a knuckle or a float with at least onepaper side weft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of reference to the drawings,in which:

FIG. 1 is a cross sectional view of a first embodiment of a formingfabric according to the invention showing the paths of one triplet ofwarp yarns in one repeat of the forming fabric weave pattern; and

FIG. 2 shows a cross sectional view similar to FIG. 1 of a secondembodiment.

In each of the schematic cross sectional views of FIGS. 1 and 2, withinthe pattern repeat the cut weft yarns shown are numbered from 1,starting with the first paper side layer weft at one side, and finishingwith the last paper side layer weft at the other. The arrows A, B and Cindicate length of the paper side layer segments in FIGS. 1 and 2. Also,in FIGS. 1 and 2 the three members shown of one triplet warp set arelabelled X, Y and Z. In both of the composite forming fabrics shown inFIGS. 1 and 2 the same weave pattern continues in each direction awayfrom the cross section shown along the length of the fabric. The weavepattern also continues across the width of the fabric, but will be movedlaterally so that the interlacing locations with the machine side layerwefts are not always with the same weft.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 is a cross sectional illustration of a first embodiment of aforming fabric according to the present invention, taken along the lineof one of the warp yarn triplets. In FIG. 1 the paper side layer of thefabric is a 2×2 plain weave, and the machine side layer is a 3×3 weave;this follows because although three warp yarns are shown in FIG. 1, eachtriplet set comprising the three yarns shown functions as a single warp.

The unbroken warp path within the paper side layer includes thefollowing three segments:

-   -   triplet Z interweaves with wefts 1; 3, 4, 6, 7 and 9, by passing        under wefts 3, 6 and 9 and passing over the others;    -   triplet X interweaves with wefts 10, 12, 13, 15, 16 and 18, by        passing under wefts 12, 15 and 18 and passing over the others;        and    -   triplet Y interweaves with wefts 19, 21, 22, 24, 25 and 27, by        passing under wefts 21, 24 and 27 and passing over the others.        Within these three segments there are three machine side layer        interlacing points:    -   triplet Z interlaces with weft 20;    -   triplet X interlaces with weft 2; and    -   triplet Y interlaces with weft 11.        These three segments with their accompanying interlacing points        then repeat with wefts 28 through 54.

The fabric of FIG. 1 is woven in 18 sheds; it could also be woven in 36.

It is thus apparent that all three members X, Y and Z of the tripletoccupy in sequence segments of the unbroken warp path in the paper sidelayer which are separated by one paper side layer weft, and all threemembers interlace alone with three regularly spaced machine side layerwefts within the length of the three paper side layer warp pathsegments.

This relatively simple weave also shows several other features of thisinvention. Inspection of the paper side layer shows that the triplets X,Y and Z follow the same path, with each one shifted along the patternrelative to the others. It can also be seen that although the spacing ofthe interlacing points is constant with two machine side layer weftsbetween each of them, the internal float lengths for each of X, Y and Zeach side of the interlacing point are not the same.

Inspection of segment A shows that triplet Z leaves the paper side layerbetween wefts 7 and 9, forms an internal float over machine side layerwefts 11, 14 and 17. In segment B, triplet Z interlaces with machineside layer 20, and forms and internal float over machine side layerwefts 23 and 26. In segment C, triplet Z re-enters the paper side layerbetween paper side layer wefts 27 and 28, interweaves with paper sidelayer wefts 28, 30, 31, 33 and 34 and Z then leaves the paper side layerbetween wefts 34 and 36. The same pattern is followed as triplet Zinterlaces with machine side weft 47. There is thus an unequal internalfloat length in triplet Z either side of wefts 20 and 47. This appliesequally to triplet X as it interlaces with wefts 2 and 29, and totriplet Y as it interlaces with wefts 11 and 38. Although the differencein internal float lengths is small, as is shown in FIG. 2 it can beavoided and yet still retain regular spacing for the interlacing points.

In FIG. 2, the paper side layer again is a 2×2 weave, with one weftbetween succeeding segments, and the machine side layer is woven to thesame 3×3 design.

The three warps X, Y and Z follow essentially the same path in the paperside layer is as is described for FIG. 1. In sequence in segment Atriplet X enters the paper side layer between paper side layer wefts 9and 10, interweaves with wefts 10, 12, 13, 15 and 16 and leaves thepaper side layer between paper side layer wefts 16 and 18. Triplet Yfollows the same path between paper side layer wefts 18 and 27, andtriplet Z follows the same path between paper side layer wefts 27 and36.

In the machine side layer although the interlacing points are regularlyspaced with two machine side layer wefts between each of them, theinterlacing points are differently located relative to the paper sidelayer so that the triplet internal float lengths are essentially thesame each side of the interlacing point. The path of triplet Z shows thedifference.

In segment A, triplet Z leaves the paper side layer between paper sidelayer wefts 7 and 9, forms an internal float over machine side layerwefts 8, 11 and 14, interlaces with machine side layer weft 17. Insegment B, triplet Z forms an internal float over machine side layerwefts 22, 23 and 26, and re-enters the paper side layer between paperside layer wefts 27 and 28. It cab thus be seen that the internal floatsin the path of triplet Z are the same length each side of itsinterlacing points with machine side wefts 17 and 44. The other twotriplets follow the same path, with equal float lengths either side ofwefts 8 and 35 for triplet Y, and either side of wefts 26 and 53 fortriplet X.

This re-location of the interlacing points provides a forming fabricwith more uniform location of the drainage openings, and a more uniformsize for the drainage openings.

Inspection of the machine side layers of FIGS. 1 and 2 shows that theinterlacing points of each of the triplets X, Y and Z can be recessed toan extent from the wear plane of the machine side layer of the fabric bythe machine side layer weft floats exposed on the machine side of thefabric, thus potentially increasing fabric life. As the exposed weftfloat length in the machine side layer weave pattern be-comes shorter,the interlacing points are recessed to a lesser degree. Wear at theselocations can thus be minimised by choosing a machine side layer weavepattern that will provide long exposed weft float lengths between theinterlacing points. It is also apparent from these diagrams thatalthough the three members of each triplet occupy in sequence thesegments of the unbroken warp path in the paper side surface, the weavepattern does not include any gaps since the pattern continues along thefabric without any breaks in either the longitudinal or transversedirections.

It is also possible to improve the protection provided for theinterlacing points by a careful choice of the yarn materials used forthe warps and wefts respectively and of the conditions under which thefabric is heat set. The yarn materials can be chosen so that the warptriplets are relatively stiffer than the machine side layer wefts, sothat the machine side layer wefts have to crimp more than the warptriplets at the interlacing points. The heat setting conditions can bechosen to achieve two objects:

(a) the stiffer warps are placed under sufficient tension to hold themrelatively straight; and

(b) the temperature is selected to promote crimping of the weftsrelative to the warps.

Typical yarn combinations and the required heat setting conditions arein Table 3.

TABLE 3 Machine Side Heat Set Heat Set Warp Layer Weft TemperatureTension PET PET/TPU about 190° C. about 805 kg/m PEN PET/TPU about 190°C. about 805 kg/mThe abbreviations for the thermoplastics yarn thermoplastic materialsare those used in Table 1.

A further benefit provided by the use of relatively high elastic moduluswarp yarns is that it is possible to diminish the size of the warp yarn.At the same yarn count, this provides a fabric with a lower warp filland higher air permeability.

As has been previously discussed, the weave structure of the paper sidelayer must “fit” onto the weave structure of the machine side layer.There are at least three reasons for this.

First, the locations at which each triplet of warp yarns interlaces witha machine side layer weft yarn must coincide with the interweavinglocation with the paper side layer of one of the other triplets. Theweave structures of each layer must therefore be such that this mayoccur without causing any undue deformation of the paper side layerpaper side surface.

Second, the paper side layer and machine side layer weave structuresshould fit such that the locations at which each triplet interlaces witha machine side layer weft is as far removed as possible from the ends ofthe segments in the paper side layer weave pattern occupied by theanother member of the triplet. This will reduce dimpling and any othersurface imperfections caused by bringing the interlacing triplet downfrom the paper side layer into the machine side layer.

Third, the locations at which each triplet interlaces with a machineside layer weft yarn should be recessed into the machine side layer asmuch as possible from the wear plane of the machine side layer, so as toextend the fabric service life. This may be accomplished by making theexposed machine side layer float between two successive interlacingpoints as long as possible. The length of a machine side layer weftfloat will increase with the number of sheds used to weave the machineside layer pattern. Thus it is generally preferred that the machine sidelayer of the fabrics of this invention be woven according to patternsrequiring at least 4 sheds, and preferably at least 6.

Experimental Trials

Four sample fabrics were woven as follows:

Sample fabric A was woven to the design in FIG. 1; and

Sample fabrics B, c and D were woven to the design in FIG. 2.

The details of these four fabric Samples are shown in Table 4.

TABLE 4 Fabric Property Sample A Sample B Sample C Sample D PS Mesh, As40.2 × 18.9 49.6 × 19.7 49.6 × 20.0 49.6 × 26.8 Woven MS Mesh, As 40.2 ×11.0 49.6 × 9.8  49.6 × 10   49.6 × 13.4 Woven PS Mesh,   45 × 17.3   55× 18.5 53.5 × 18.1 56.7 × 25.2 Heatset MS Mesh,  45 × 8.7  55 × 9.3 53.5× 9   56.7 × 12.6 Heatset Warp Diameter  0.25 mm  0.20 mm  0.20 mm  0.20mm Warp Material PET PEN PEN PEN PS Weft  0.26 mm  0.22 mm  0.22 mm 0.18 mm Diameter PS Weft PET PET PET PET Material MS Weft  0.45 mm 0.45 mm  0.45 mm  0.30 mm Diameter MS Weft PET/TPU PET/PA-6 PET/PA-6PET Material PS Weave Plain Weave MS Weave 1/8 Float Heatset Approx.200° C. Temperature Cloth Elastic 2590 kg/cm 1744 kg/cm 2068 kg/cm 1846kg/cm Modulus Fabric Caliper 0.019 mm 0.017 mm 0.0165 mm 0.0146 mm MSWeft −0.0059 0 0 −0.0044 Crimp Warp Fill 100% 100% 100% 100% as wovenWarp Fill, 110% 110% 110% 110% Heatset Fiber Support 84 Index (Beran)Air 7,890 10,300 8,210 8,690 Permeability Notes to Table 4. PS: paperside layer. MS: machine side layer. Mesh: warp × weft per cm. PET, PEN,PA6 and PET/TPU: see Table 1. PET/PA-6: Alternating yarns of PET andPA-6. Air Permeability: m³/m²/hour; measured on the heat set fabric byASTM D 737-96 using high pressure machine as available from Frazier HighPrecision Instrument Co., Gaitherburg, MA, USA, at a pressuredifferential of 127 Pa through the fabric. Elastic Modulus of Cloth:slope of a stress-strain curve at a tension of from 3.6 kg/cm to 7.1kg/cm in a CRE type tensile testing machine. Caliper: average of atleast 5 thickness measurements. MS Weft Crimp: the amount by which theknuckles of the machine side layer weft yarns lie above (negative value)or below (positive value) the plane of the machine side layer warps.Warp Fill: (warp diameter × mesh × 100)%, Fiber Support Index:determined according to the relationship provided in CPPA Date SheetG-18 and refers to amount of support provided by the paper side surfaceof the paper side layer available to support the papermaking fibers inthe stock deposited thereon.

Inspection of Table 4 shows that although the elastic modulus of SampleA was significantly higher, this fabric also has the highest caliper,due at least in part to the yarn materials used in it. The fabrics ofSamples A and D both show a negative MS weft crimp, which indicates thatin these fabrics a good wear life can be expected due to the outwardbowing of the long floats in the machine side layer weave design. Thiswear life is also enhanced by the use of the PET/TPU material in themachine side layer weft yarns.

Selection of appropriate warp and weft yarn diameters for use in thefabrics of this invention will depend on many factors, including thegrade of paper product which the fabric will be used to produce and willaffect the air permeability of the resulting fabric. Selection ofappropriate yarn diameters will be made in accordance with the intendedend use of the fabric.

Table 4 shows that the fabrics of this invention possess good airpermeability, of from 10,300 down to 7,890 m³/m²/hr in the samplefabrics for which data is given in Table 4. Fabric air permeability maybe further reduced by appropriate choice of paper side and/or machineside yarn diameter and mesh. By reducing fabric air permeability, fluiddrains more slowly through both the paper and machine side fabriclayers, which result in improved formation and reduced wire mark.Laboratory analysis of hand sheets produced on the fabric samplesdescribed in Table 4 confirms that wire mark is reduced compared toother prior art fabrics, and that the sheets offer improved printabilitycharacteristics.

1. A composite forming fabric having a paper side layer and a machineside layer, which comprises: (i) a first set of paper side layer weftyarns, (ii) a second set of machine side layer weft yarns which arelarger than the paper side layer weft yarns, and (iii) a set of tripletwarp yarns which contribute to the structure of both the paper sidelayer and the machine side layer, which three sets of yarns are woventogether according to a repeating pattern wherein: (a) each member ofeach triplet set of warp yarns interweaves with the paper side layerweft yarns to occupy in sequence segments of a single unbroken warp pathin the paper side layer; (b) the sequence of segments repeats as part ofthe repeating pattern; (c) each segment in the unbroken warp path isseparated next segment by at least one paper side layer weft yarn; (d)each member of each triplet interlaces separately with a single machineside layer weft yarn at least once within the pattern repeat; (e) withinthe fabric repeating pattern the number of machine side layer weft yarnsbetween each interlacing point of successive yarns from each triplet ofwarp yarns is constant; and (f) within the fabric repeating pattern thepath lengths of each member of each triplet set is the same.
 2. Aforming fabric according to claim 1 wherein the fabric as woven andprior to heat setting has a warp fill of from 100% to 125%.
 3. A fabricaccording to claim 1 wherein the warp and weft yarns are thermoplasticmonofilaments.
 4. A fabric according to claim 3 wherein the first andsecond set of weft yarns and the warp yarns are all monofilaments of thesame thermoplastic.
 5. A fabric according to claim 4 wherein the warpyarns and the first and second sets of weft yarns are all polyethyleneterephthalate monofilaments.
 6. A fabric according to claim 3 whereinthe first set of weft yarns, the second set of weft yarns and the warpyarns is are not all monofilaments of the same thermoplastic.
 7. Afabric according to claim 3 wherein the first set of weft yarnscomprises at least a first and a second subset of weft yarns and eachsubset comprises monofilaments of different thermoplastics.
 8. A fabricaccording to claim 3 wherein the second set of weft yarns comprises atleast a third and a fourth subset of weft yarns and each subsetcomprises monofilaments of different thermoplastics.
 9. A fabricaccording to claim 1 wherein the warp yarns are thermoplasticmonofilaments having a higher modulus of elasticity than the machineside layer weft yarn thermoplastic monofilaments.
 10. A fabric accordingto claim 9 wherein the ratio of the moduli of elasticity of the warpyarns and the machine side layer weft yarns is about 4:3.
 11. A fabricaccording to claim 1 wherein within each of the first set of weft yarns,the second set of weft yarns, and the warp yarns, the yarns are all ofthe same size.
 12. A fabric according to claim 3 where in the first setand the second set of weft yarns are polyethylene terephthalatemonofilaments.
 13. A fabric according to claim 1 wherein the second setof weft yarns are yarns chosen from the group consisting of polyethyleneterephthalate monofilaments, monofilaments of a blend of polyethyleneterephthalate and a thermoplastic polyurethane; polyamide monofilamentsand mixtures thereof.
 14. A fabric according to claim 13 wherein in thesecond set of weft yarns the third subset comprises monofilaments of ablend of polyethylene terephthalate and a thermoplastic polyurethane,the fourth subset are yarns chosen from the group consisting ofpolyethylene terephthalate monofilaments, polyamide monofilaments andmixtures thereof, and the third subset comprises at least 50% of theyarns in the second set in the machine side layer.
 15. A fabricaccording to claim 3 wherein the warp yarns are chosen from the groupconsisting of polyethylene terephthalate monofilaments, polyethylenenaphthalate monofilaments, and mixtures thereof.
 16. A fabric accordingto claim 13 or 14 wherein the polyamide monofilaments are chosen fromthe group consisting of polyamide-6 and polyamide-6/6 monofilaments. 17.A forming fabric according to claim 1 wherein the fabric has an airpermeability, when measured by a standard test procedure, of from lessthan about 7,500 m³/m²/hr, to about 11,000 m³/m²/hr at a pressuredifferential of 127 Pa through the fabric.
 18. A forming fabricaccording to claim 1 wherein the paper side layer weave design is chosenfrom the group consisting of a 2×2, 3×3, 3×6 or 4×8 weave design.
 19. Afabric according to claim 18 wherein the paper side layer weave designis chosen from the group consisting of a plain 2×2 weave, a 3×3 weave,and a 4×4 weave.
 20. A fabric according to claim 1 wherein the machineside layer weave design is chosen from the group consisting of a 3×3,4×4, 4×8, 5×5, 6×6 or 6×12 weave design.
 21. A fabric according to claim20 wherein the weave design of the machine side layer is chosen from a3×3 twill, a 6-shed broken twill, or an N×2N design as disclosed byBarrett in U.S. Pat. No. 5,544,678.
 22. A fabric according to claim 1wherein the ratio of paper side layer weft yarns to machine side layerweft yarns is chosen from 1:1, 2:1, 3;2, 5;3 and 3:1.
 23. A fabricaccording to claim 22 wherein the ratio of paper sidle layer weft yarnsto machine side layer weft yarns is about 2:1.
 24. A fabric according toclaim 1 wherein the quotients expressed as Q/P and Q/M, in which Q isthe total number of sheds, P is the number of sheds required to weavethe paper side layer design, and M is the number of sheds required toweave the machine side layer design are integers.
 25. A fabric accordingto claim 3 wherein the warp yarns are polyethylene naphthalate, thefirst set of weft yarns are polyethylene terephthalate, and in thesecond set of weft yarns the third subset comprises monofilaments of ablend of plyethylene terephthalate and a thermoplastic polyurethane andthe fourth subset comprises polyamide monofilaments.
 26. A fabricaccording to claim 3 wherein the warp yarns are polyethyleneterephthalate, the first set of weft yarns are polyethyleneterephthalate, and in the second set of weft yarns the third subsetcomprises monofilaments of a blend of plyethylene terephthalate and athermoplastic polyurethane and the fourth subset comprises polyamidemonofilaments.